Interpretive Summary: Between harvest and the start of drying, paddy rice may be held for more than 24 hours at moisture contents ranging from 16% to more than 26%. Microbes (fungi, bacteria, etc.) found on the freshly harvested rice grow under these conditions and produce a wide variety of compounds which impact the flavor/aroma of the white rice obtained following drying and milling of the paddy. The contents of ten compounds produced by the microbes were compared in white rice obtained from paddy rice harvested at differing moisture contents and immediately dried (0 hr) or held for 48 hr prior to drying. No increases in the amounts of these compounds were observed in white rice obtained from paddy which was stored at 17 to 21% moisture contents for 48 hr. In white rice from paddy stored at 24% moisture content and higher, compounds which cause flavor and odor problems increased markedly with time. A sensory panel found that sour/silage and alfalfa/grassy/ green bean flavors increased in intensity. The intensity of sour/silage flavor was found to be related to the amount of ethanol and 2,-3-butandiol produced by the microbes. Measurement of ethanol produced in paddy rice may serve as an indicator of off-flavor/aroma development in the resultant white rice. This research will assist the rice industry in being able to predict, based on the paddy moisture content, how long the wet rice can be held before off-flavor/aroma problems occur.

Technical Abstract:
Between harvest and the start of drying, paddy rice may be held for more than 24 hours at moisture contents ranging from 16% to more than 26%. Microbes found on the freshly harvested rice grow under these conditions and produce a wide variety of volatile compounds which impact the flavor/aroma of the white rice obtained following drying and milling of the paddy. The contents of ten volatile microbial metabolites were compared in white rice obtained from paddy rice harvested at differing moisture contents and immediately dried (0 hr) or held for 48 hr prior to drying. No increases in volatile compound levels were observed in white rice obtained from paddy which was stored at 17 to 21% moisture contents for 48 hr. In white rice from paddy stored at 24% moisture content and higher, 3-methyl-butanol, 2-methyl-butanol, acetic acid, 2,3-butandiol, and ethyl hexadecanoate increased markedly with time. Also in these samples, as determined by a descriptive panel, sour/silage and alfalfa/grassy/ green bean flavors significantly increased (P < 0.1) in intensity. Sour/silage, the predominant off-flavor note in the stored samples, correlated highly ( r = 0.98) with 2,3-butandiol. Ethanol concentration measurements on the paddy correlated highly with sour/silage ( r = 0.99) and well with several other volatile compounds. Carbon dioxide measurements taken on the paddy rice did not correlate as highly (r = 0.78) with sour/silage. Measurements of ethanol produced in paddy rice may serve as an indicator of off-flavor/aroma development in the resultant white rice.